RAB23 Modulates Signaling Protein Ciliary Homeostasis through Promoting RAB18-mediated Inward BBSome Transition Zone Passage

As an intraflagellar transport (IFT) cargo adaptor, the BBSome targets the basal body region located just below the transition zone (TZ) before proceeding to cross the TZ for ciliary entry. This process of TZ crossing crucially governs BBSome presence and quantity available for signaling protein coupling within cilia, influencing the dynamic behavior of signaling proteins in cilia by controlling their ciliary removal. Our previous study revealed that the GTPase RAB18 binds to its BBSome effector in the basal bodies right below the TZ and recruits the BBSome to cross the TZ for ciliary entry by lateral transport between the plasma and ciliary membranes. However, the mechanism by which RAB18 is activated to accomplish this function has remained elusive. This study uncovers that, under physiological conditions, Chlamydomonas RAB23 in its GDP-bound state (RAB23-GDP) predominantly localizes to the same basal body region as RAB18-GDP and activates RAB18-GDP as a RAB18-specific guanine nucleotide exchange factor. RAB18-GDP attaches to the membrane and binds to the BBSome. This interaction occurs independently of anterograde IFT association, allowing the BBSome to diffuse through the TZ and enter cilia. Subsequently, RAB23-GDP facilitates its own ciliary entry by binding to the RAB18-GTP/BBSome entity as a BBSome cargo. Our findings establish a model where the BBSome enters cilia through the RAB23-RAB18 module-mediated inward BBSome TZ diffusion pathway. According to this model, hedgehog signaling defects caused by rab23 mutations could induce Carpenter syndrome in humans, providing a mechanistic understanding behind the inward BBSome TZ passage required for proper ciliary signaling.